Modified Project Summary/Abstract Section The Nucleus Accumbens (NAc) is a key brain region mediating mood-relevant behaviors but the cell types and molecules underlying this contribution are still being identified. Additionally, many NAc-relevant behaviors show sex differences, but we lack insight into the mechanisms mediating these differences. Studies on the molecular and cellular bases of sex differences in NAc function and behavior will fill a key knowledge gap by identifying novel targets and mechanisms. The long-term goal of this project is to determine the neurobiological mechanisms of activity-regulated cytoskeleton-associated protein (Arc) action in the adult NAc, focused on Arc?s role in mediating sex-specific cellular and behavioral NAc functions via cell type-specific action. The overall objective of this proposal is to determine the role and regulation of Arc in the NAc of male and female mice following NAc-related behavioral experiences (e.g. stress, motivation, novelty exposure). The central hypothesis is that Arc regulates NAc function and associated behaviors via cell type-specific synaptic action. The rationale for this project is that the identification of specific cell types mediating NAc-related behaviors, and Arc?s function in those cells, directs future hypotheses on the cellular and synaptic mechanisms of sex differences. The central hypothesis will be tested with two specific aims: 1) Establish the cell types inducing Arc following NAc-relevant behavioral experience and requiring Arc action to mediate behavioral responses; 2) Determine the subcellular localization of Arc and its role in synaptic plasticity in the NAc. In the first aim, key cell populations inducing Arc in response to NAc-relevant behavior experience will be identified using immunohistochemical techniques directed against Arc protein and genetically encoded, cell type-specific fluorophores. The cells requiring Arc action to mediate NAc-relevant behavior will be identified with in mice expressing genetically encoded cell type-specific cre, with cre-dependent shRNA virus delivery to the adult NAc. In the second aim, the subcellular localization of Arc will be assessed immediately following behavior experience using biochemical fractionation techniques. Additionally, Arc?s contribution to NAc synaptic function will be assessed using electrophysiological techniques to examine basal physiology and the ability to induce NAc-relevant, Arc-mediated forms of experimenter-induced synaptic plasticity (e.g. BDNF-induced LTP, DHPG induced-LTD). The research proposed in this application is innovative, as it is focused on examining a molecular mediator of NAc function, localizing roles for Arc in specific cell types within the NAc and connecting Arc action to cellular and behavioral sex differences. The proposed research is significant as it establishes a novel role for a single molecule (i.e. Arc) in specific cells of a single brain region (NAc) as mediator of sex differences in NAc function and behavior. The proposed experiments provide critical evidence for the relevance of Arc in NAc-mediated behavior, sex differences in NAc function and behavior, and critical information for future experiments to delineate the sex-specific regulation and function of Arc.